6.1.4—The formula for the spring constant k = F / x

The spring constant is k=F/x in the proportional region. Its unit is N m⁻¹; a larger k means more force is needed for the same extension.

Convert extension to metres before calculating. Compare like-for-like loading conditions and do not use points beyond proportional behaviour.

A 12 N force producing 0.040 m extension gives k=300 N m⁻¹.

k is not the extension itself, and a stiffer spring has a larger k, not a larger extension under the same force.